Annual post-harvest spoilage of highly perishable agricultural commodities such as fruits has been estimated to result in losses in the range of 20-50% of the crop worldwide, depending on the sophistication of available storage facilities.
Fruits are generally susceptible to rapid post-harvest degradation due to high respiration rates and microbial spoilage. Upon harvesting, fruits undergo enzymatic breakdown, becoming soft or shriveled. This is regarded as normal post-harvest deterioration and is acceptable up to a certain level. Unacceptable post-harvest deterioration is the occurrence of diseases caused mainly by fungi, which require a special antifungal treatment. Synthetic fungicides are currently the main agents for controlling post-harvest wastage due to pathogenic fungi and have been used extensively for decades.
Post-harvest treatments aimed at delaying deterioration include washing, waxing, curing, plant growth regulators, sprout inhibitors, disinfectants, irradiation, controlled ripening, controlled de-greening, light (minimal) processing, and chemical treatments.
Chemical treatments to reduce post-harvest decay have only become of significant use in the last 30 years. The success of these treatments depends on the initial spore load, the time that passes from infection to treatment, the depth of the infection within the host tissues, the growth rate of the infection, temperature and relative humidity, and the depth at which the chemical can penetrate the host tissues.
In addition, the applied chemical must not be phytotoxic, must comply with local food additive laws and must have established maximum residual limits (MRLs). Chemicals may be impregnated into wraps, box liners, applied as fumigants, solutions and suspensions, or in wax.
Most of the post-harvest treatments currently available are aimed at treating citrus fruits, pome fruits such as apples, pears or quinces, and melons or bananas. Very little success has been reported in treating soft fruits such as strawberries, raspberries, or grapes, and persimmons or peaches.
Today, the agricultural industry uses a variety of post-harvest antifungal treatments based on synthetic chemicals, to avoid or delay disease and prolong the shelf life of fresh produce. However, fungicide-tolerant strains, e.g. Botrytis spp., Penicillium spp., are present in most packing houses, rendering synthetic fungicides less effective or totally ineffective (Spotts et al, 1986).
Another problem concerning the use of synthetic fungicides is the fact that they leave residues, which may be considered carcinogenic and/or environmentally hazardous.
In addition, in the organic agriculture sector no post-harvest treatment is applied, as no product currently meets the stringent standards of international organic growers' associations. With the ever-increasing popularity of organically grown produce, there is an urgent demand to supply this sector with an acceptable treatment that will lengthen the storage and shelf life of fresh agricultural produce.
The potential for post-harvest application of synthetic fungicides is limited also by both adverse effects due to wetting of the fruits and stringent regulations concerning the use of currently available fungicides. This issue has contributed to an urgent and significant need to develop alternative, safe, environmentally benign, and effective methods for controlling post-harvest pathogens, capable of complementing, or even completely replacing synthetic fungicides.
Biological control methods, involving a range of different approaches, including strengthening the commodity's natural defense mechanisms and application of antagonistic microorganisms and natural antimicrobial substances, have become popular in recent years.
Naturally occurring biologically active secondary metabolites from plants are examples of antimicrobial and antifungal compounds and may offer a new and effective solution for control of post-harvest diseases of horticultural products.
Essential oils, a class of volatile oils extracted from plants, fruits or flowers by steam, distillation or solvent extraction, are known to possess antimicrobial activity. Extensive work is being carried out around the world on essential oils and their components for several purposes: as pharmaceuticals for the drug industry or for non-conventional natural therapy, for cosmetic purposes; in agriculture, as pesticides or for preservation of foodstuff and agricultural produce.
Numerous essential oils produced by different genera are endowed with allelopathic, antimicrobic, antioxidant and/or bioregulatory properties (Deans et al, 1992; Piccalgia et al, 1993). The use of essential oils as antifungal agents in post-harvest storage is very promising owing to their negligible toxicity to mammals. In addition, due to their high degree of volatility, they can be used in active packing or in cold storage. Both the Council of Europe's Committee of Experts on Flavoring Substances and the FDA have not recommended limits on the use of essential oils. Furthermore, essential oils and their components have been evaluated by the US Flavoring Extract Manufacturers Association (FEMA) as GRAS (Generally Regarded As Safe).
The pesticidal activity of essential oils or components thereof in general is known from the literature and several essential oils have been proposed for use as plant pesticides. The fungicidal or fungistatic activity of essential oils in general is known from the literature for the protection and preservation of foodstuff (e.g. bread, meat), cereals (e.g. wheat) and agricultural commodities (Wilson et al, 1987).
Prior studies concern mainly in vitro testing (under ideal conditions) of essential oils or their components against fungi known to cause damage to fruits. Only a small number of studies have tested essentials oils in vivo (under commercial conditions) in post-harvested fruits and vegetables.
Different essential oils exhibit different fungicidal and fungistatic activity. In vitro, each essential oil is active against some types of fungi and is not active against others. Different essential oils may be active against the same pathogen, but at different concentrations. In vivo, the essential oil may be active against a certain pathogen in one type of fruit, but not against the same pathogen in a different fruit, or at least not at the same concentration.
The actual mode of action of essential oils is not fully known, although it is speculated to involve membrane disruption of pathogenic fungi by the lipophilic compounds (Bennis et al, 2004). Essential oils cause degeneration of the fungal hyphae, which are emptied of their cytoplasmic content (Zambonelli et al, 1994.) Destruction of the fungal hyphae prevents the spread of mycelia into new tissue, while the sporangia cannot sporulate to form new infective spores. Spores that have already spread and are dormant, awaiting favorable conditions for germination, may also be affected in the same way (Lambert et al, 2001).
The biological activity of the essential oils is evidently due to synergistic action of their components. Complex mixtures of the individual components (mostly monoterpenes and sesquiterpenes) in the oils are far more potent as a barrier to pathogen adaptation than the individual component (Carlton et al, 1992).
Some post-harvest pathogens have a limited host range. For instance, Penicillium digitatum attacks only citrus fruits and causes a green mold. Other pathogens are omnivorous and have a wide host range. Omnivorous fungi include Alternaria alternata, Botrytis cinerea, and Rhizopus spp., etc. All these fungi are economically important pathogens of a wide range of fruits.
Currently, green mold of citrus, caused by Penicillium digitatum, is controlled by applying synthetic fungicides such as imazalil and thiabendazole. Black spot decay of persimmon fruits is caused by Alternaria alternata, which attacks the developing fruits, but infections remain quiescent until after harvest, when the symptoms become apparent following prolonged storage at low temperature. A postharvest dip in a hypochlorite solution provides a certain degree of control (Prusky et al., 2001), but only storage using modified atmosphere packaging (MAP) has resulted in a sufficient delay in fungal development (Ben-Arie et al., 1991). MAP also decreases strawberry fruits decay caused by Botrytis cinerea but contributes to off-odors and flavors (Shamalia et al., 1992). Effective chemical control of Rhizopus rot in peaches is provided by dichloran. But its use was discontinued due to visible residues on the fruits. Moreover, the widespread use of chemicals in commercial packing houses has led to the proliferation of resistant strains of many pathogens (Palou et al, 2001).
Recent research has focused on the search for novel compounds active against these fungi, which may have potential in disease control. Some natural compounds have already been identified. Caccioni and Guizzardi (1994) reported that several oil extract components inhibited germination and growth of a wide range of fruits and vegetable post-harvest pathogenic fungi. Oil extracts from oregano (Thymus capitatus) (Arras et al., 1995); sage (Salvia officinalis) (Carta et al., 1996); marjoram (Oreganum syriacum), lavender (Lavandula angustifolia), lemongrass (Cymbopogon citratus), tea tree (Melaleuca alternifolia), melissa (Melissa officinalis), peppermint (Mentha piperita), penny royal (Mentha pulegium), jasmine (Jasminum grandiflorum), bois de rose (Rosa spp.), neroll (Citrus aurantum), wintergreen (Gaultheria procumbens) and hyssop (Hyssopus officinalis) (Cutler et al., 1996), all inhibited in vitro mycelial growth of Botrytis cinerea. However, the efficacy of these extracts may differ in in vivo systems under the influence of environmental conditions. Phytotoxicity problems may arise when these extracts are applied in liquid form to living fruit tissues.
Several publications and patents disclose the use of essential oils for food preservation. Japanese Publication No. JP59132876 discloses the use of a mixture of ethyl alcohol and an essential oil, essential oil component, spice or spice component such as allyl mustard oil, or beefsteak plant oil supported on a carrier such as non-woven cloth and zeolite, for food products and vegetable preservation inside a container that is sealed with a non-permeable material.
Japanese Publication No. JP58101670 discloses the use of a volatile substance with preservative effect such as ethyl alcohol, an organic acid or an essential oil, on a pouch made of a macromolecular continuous film with controlled permeability, for food preservation, wherein the pouch and the food product are packed together and sealed tightly. Japanese Publication No. JP58063348 describes vegetables that can be protected from microbial deterioration by adsorbing a substance such as an essential oil on zeolite, and packing the vegetables with the resulting zeolite. Japanese Publication No. JP8205768 discloses the use of thymol or Thyme essential oil for preservation of freshness of mushroom. No pathogens are mentioned.
Moldovian Patent No. MD682 discloses the use of Origanum heracleoticum essential oil (0.4-0.5%) in mixture with ethanol (99.5-99.6%) for protection of fruits (grapes) from decay caused by pathogenic fungi.
Japanese Publication No. JP10179104 discloses the use of one or more essential oils (cinnamon, garlic, thyme, oregano) for preservation of food (bread, dairy, fish, cakes, etc.), also against pathogenic fungi.
PCT Publication WO 00/21364 describes essential oils from plant species of the families Labiatae or Umbelliferae (e.g. Origanum, Thymbra, Pinpinella) that protect plants from insects, fungi, nematodes and bacteria when applied to the soil, leaves, etc.
U.S. Pat. No. 5,958,490 describes the use of activated carbon impregnated with essential oils and benzaldehyde, to provide controlled release of volatiles for the control of post-harvest disease. Neither pathogens, nor essential oils are specified.
U.S. Pat. No. 6,482,455 discloses the use of a composition consisting of the association of thymol, eugenol and cinnamaldehyde and an oligosaccharide, for the control of post-harvest pathogens of fruits and vegetables, applied in solid form (incorporated into the wax) or in a diluted dip. No pathogens are mentioned.
The efforts for finding essential oils for protection of fruits from pathogenic fungi without blemishing the fruits or affecting their taste and aroma have not been successful as yet.